Indices of climate change

Two indices have been proposed to monitor regional changes in climate (1). The indices are based on factors that matter to human comfort and the economy. For instance they include rainfall but not the height of the tropopause. They were developed for the USA but can be applied elsewhere.

• The Climate Extremes Index CEI is based on various indicators of extreme rainfall and temperature events. One of the five factors contributing to the CEI, for instance, is the percentage of the area of the USA with 'extreme' annual-mean maximum temperatures, i.e. either much below or much above normal. The five factors are averaged to obtain the CEI.
• The Greenhouse Climate Response Index GCRI is calculated from factors that might be expected to alter with any increase of greenhouse gases in the atmosphere. The GCRI is similar to the CEI, with five rainfall and temperature factors. One of them is the areal percentage with annual mean temperature much above the longterm mean.

Changes of the CEI this century suggest that extreme rainfalls and temperatures are more frequent in recent decades in the USA, but the evidence is but inconclusive. More specifically, there has been an increase in the fraction of the total rainfall at any place due to the most extremely heavy falls there. Also a decrease of the area with maximum temperatures much below the longterm average. Cold season rainfall has increased, and rainfalls have become more frequent.

The CEI shows similar maxima in the 1930’s and around 1952, with a plateau since 1970. Likewise, there are indications of an increase of GCRI. Both indices changed abruptly during the 1970’s, when there were major changes of atmospheric circulations over the Pacific Ocean and North America. The GCRI does not correlate with the annual-mean Southern Oscillation Index.

(1) Karl, T.R., R.W. Knight et al. 1996. Indices of climate change for the United States. Bull. Amer. Meteor. Soc., 77, 279-92.